@Otherbrother, because i had to look…
project implicit is a harvard sponsored nonprofit that consults on with implicit bias in organizations and education. they built a test to (try to) measure biases [ see vox’s piece below for why it works better in the aggregate than at an individual level ]
they did a big public internet study, and that’s where the graph is likely pulled from… though i wasn’t able to quickly find the textbook 
This data archive includes Race Implicit Association Test (IAT) scores of 2,355,303 Internet volunteers who completed educational/demonstration versions of the Race IAT at https://implicit.harvard.edu from 2002 to 2012.
vox had a fairly big piece on it about five years ago:
it probably had big academic buzz at the time, and i’d guess is when it got added to the textbook
